Lead acid batteries were first developed more than 150 years ago, and are recognized as the first rechargeable battery. Despite the development of many other battery technologies over the past 150 years, lead acid batteries still enjoy wide use today in a number of industries, especially for automobile starting, lighting and ignition (SLI) applications. While lead acid batteries have a relatively low energy-to-weight and energy-to-volume ratio compared to other types of battery systems, their ability to supply large amounts of current in short bursts give them one of the highest power-to-weight ratios of any battery system. The ability of lead-acid batteries to deliver high amounts of surge current to components like automobile starter motors have kept them an important part of the battery industry for many decades.
Lead acid batteries are also unusual among battery systems for using the same element, lead (having chemical symbol Pb), as the electro-active material in both the positive and negative electrodes of the battery. You can see an illustration of this in FIGS. 1A and 1B, which show diagrams of the chemical reactions taking place during the charging and discharging of a lead acid battery that uses an aqueous sulfuric acid solution as the electrolyte. The use of lead in both the positive and negative electrodes is possible because the lead compounds have different oxidation states in each electrode. The negative electrode includes lead metal, Pb(s), in the zero oxidation state (Pb0) that is oxidized to Pb2+ when the battery is discharged. In contrast, the positive electrode includes lead oxide, PbO2(s), in the plus-four oxidation state (Pb4+) that is reduced to the same Pb2+ during battery discharge. The electrolyte in lead-acid batteries is aqueous sulfuric acid [H2SO4 (aq)], and the sulfate ions (SO42−) in the electrolyte counterbalance the Pb2+ ions formed during discharge at both the negative and positive electrodes to make solid lead sulfate (PbSO4(s)). The conversion of Pb(s) to PbSO4(s) at the negative electrode and the complementary conversion of PbO2 to PbSO4(s) at the positive electrode are reversed when a recharging current is supplied to the lead-acid battery, for example through the alternator of an automobile. The complete half reactions for the positive and negative electrodes can be written as follows:Pb(s)+HSO4−(aq)↔PbSO4(s)+H+(aq)+2e−  (Neg Electrode)PbO2(s)+HSO4−(aq)+3H(aq)+2e−↔PbSO4(s)+2H2O(l)  (Pos Electrode)
Unfortunately, the electrode half reactions shown above are not the only reactions that can occur in a lead-acid battery: When a lead acid battery is charged too quickly, or continues to be charged at too high a voltage after reaching full charge, the electric current changes from (i) electrolyzing the lead sulfate back into lead (Pb(s)) and lead oxide (PbO2(s)) to (ii) hydrolyzing the water in the electrolyte to hydrogen (H2(g)) and oxygen (O2(g)) gas. Not only does this reduce the amount of water present in the electrolyte, it also causes the buildup of an explosive gas mixture within the battery. If the gases are vented without replenishing the water, the battery could run dry resulting in the electrodes being permanently damaged or destroyed. Even worse, if the gases are not vented quickly enough to prevent a buildup of pressure, the pressurized hydrogen and oxygen gases could explode.
Another undesired reaction is the irreversible formation of large lead sulfate crystals on the electrodes in a process known as sulfation. These larger crystals of lead sulfate act as electrical insulators that attenuate and eventually stop electrical conduction though the battery's electrodes. Sulfation is most prevalent in batteries that are undercharged or slowly charged to give fine particles of lead sulfate a chance to act as seed crystals for the growth of larger lead sulfate crystals that cannot be eliminated by applying more charge. It is the most common cause of premature lead acid battery failure.
Lead acid battery developers have incorporated additives into the electrolyte solution that suppress the electrolysis of water from the electrolyte and the formation of large lead sulfate crystals on the electrodes. These additives have been shown to reduce maintenance costs and extend the lifetime of lead acid batteries. However, many of the additives themselves have lifetimes significantly shorter than the battery due in part to the highly reactive and corrosive environment of a concentrated sulfuric acid solution. Thus there is a need to develop new additives that are stable for extended periods of time in the difficult environment of a lead acid battery. There is also a need to develop new materials and methods that can introduce the additives to the electrolyte over an extended period of time. These and other challenges are addressed in the present application.